1. Field of the Invention
Embodiments of the present invention relate, in general, to systems and methods for frequency use and allocation in a wireless network and particularly to reliable channel sensing in wireless regional area network cells employing dynamic frequency hopping.
2. Relevant Background
Cognitive radio is considered to be an enabling technology that allows unlicensed radio transmitters to operate in licensed bands at locations where that spectrum is temporally not in use. Based on cognitive radio technology, IEEE 802.22 is an emerging standard for wireless regional area networks (WRANs) operating on a license-exempt and non-interference basis in the spectrum allocated to TV broadcast services (between 47-910 MHz). This standard aims at providing alternative broadband wireless Internet access in areas without creating harmful interference to licensed TV broadcasting.
A WRAN cell consists of a Base Station (BS) and the associated Customer Premise Equipments (CPEs) that communicate to the BS via a fixed point-to-multi-point radio air interface. The typical radius of the coverage area is on the order of 33 km. WRAN operations need to satisfy two apparently conflicting requirements: (1) assure the Quality of Service (“QoS”) satisfaction for WRAN services, and (2) at the same time, provide reliable and timely spectrum sensing for guaranteeing the licensed user protection.
Dynamic Frequency Hopping (“DFH”) is a technique that incorporates non-traditional dynamic channel allocation with slow frequency hopping. The main objective in DFH is to provide capacity improvements through the addition of interference avoidance that are higher than those provided by conventional frequency hopping while preserving interference averaging characteristics of conventional frequency hopping in order to provide robustness to rapid changes in interference.
Generally, the key concept behind this intelligent type of frequency hopping is to adjust or create frequency hopping patterns based on interference measurements. DFH uses slow frequency hopping and adaptively modifies the utilized frequency hopping pattern based on rapid frequency quality measurements, also referred to as QoS measurements. This technique combines traditional frequency hopping with dynamic channel allocation, where a channel is one frequency in a frequency hop pattern. The continuous modification of frequency hop patterns is based on measurements representing an application of dynamic channel allocation to slow frequency hopping. Modifications are based on rapid interference measurements and calculations of the quality of frequencies used in a system by all CPEs and BSs. The target of these modifications is tracking the dynamic behavior of the channel quality as well as of interference.
Current sensing requirements state that incumbent signals shall be detected by WRAN devices with no more than a 2 second delay. Within IEEE 802.22, the sensing mechanism is designed to offer protection to two types of incumbents, namely, the TV service and wireless microphones. Analyses of well-known sensing technologies show that the sensing task takes up to several tens of milliseconds per channel, given the required reliability. For example, the Digital Television (“DTV”) energy detection at 6 MHz requires 69.43 ms per channel. In fact, because of out-of band interference, a channel can be considered to be free only if its adjacent channels are also free, making it necessary to sense several channels. Hence, a sensing period can range from tens of milliseconds up to more than 100 milliseconds. Incumbent signals (DTV and the like) must be detected by WRAN devices starting from the time the licensed signal exceeds the detection threshold on a TV channel. Thus a WRAN cell must perform sensing on a working channel at least every 2 seconds. Unfortunately a channel that is to be sensed cannot be used for data transmission, thus a cell operating consistently on a single channel must interrupt data every 2 seconds for sensing. Such a non-hopping mode leads to periodic interruptions and can significantly decrease system throughput and impair QoS. The solution to this problem, as will be appreciated by one skilled in the art, is DFH.
As previously described, DFH differs from conventional frequency hopping in the way the patterns are built. Instead of using random or pre-defined repetitive hopping patterns, DFH patterns are dynamically generated for active users. In this manner, the hopping patterns can be adjusted to adapt to interference changes. The basic idea behind creating the patterns is to choose the best frequency for each hop. This best frequency corresponds to the frequency that is interfered with the least. DFH thus requires continuous estimation and measurement (sensing) of the interference at every frequency for every single hop of a pattern.
Spectrum sensing of this type takes time away from periods that could otherwise be used for data communication. Hence sensing can compromise the system's QoS. It is thus critical to analyze and understand the interplay of sensing and spectrum management and how best to optimize them for different situations that may occur during DFH.